G protein coupled receptors play a key role in cellular communication in cardiovascular, endocrine, and neural signaling as well as in cell growth and differentiation. The specific conformational changes associated with activation of G protein coupled receptors are poorly understood at a molecular level. Mutations in these receptors which lead to constitutive activation have been identified in a small number of human diseases and it is likely that this mechanism is at play in other more common disorders. At a molecular level these constitutively activating mutations provide a set of powerful tools to better understand receptor activation. A major gap in our understanding of the molecular mechanisms of cellular communication derives from the paucity of high resolution membrane protein structures. Solid-state NMR has emerged recently as a powerful approach to obtain high resolution structural information about membrane proteins. We propose to examine by solid-state NMR, the structure of bacterially expressed 15N labeled receptor fragments which contain transmembrane domains and the critical G protein activating structures. The effect of constitutively activating mutations on the mobility and orientation of both the transmembrane and cytoplasmic structures will be determined to begin to establish the molecular basis for G protein activation.